The present invention relates to a vehicular two-wheel/four-wheel drive switching device for switching between two-wheel and four-wheel drive modes of operation.
Heretofore, vehicles capable of switching between two-wheel and four-wheel drive modes of operation have been known. FIGS. 4 and 5 show an example of such a vehicle. The vehicle 1 has a body frame 3 in the center of which an engine 2 is mounted. A pair of front wheels 4 and a pair of rear wheel 5 are arranged at the front and rear portions of the body frame 3, and a steering handlebar 6 is arranged above the front portion of the body frame 3 for steering the vehicle via the front wheels 4. A fuel tank 7 is located above the engine 2 and attached to the body frame 3, and a seat 8 is attached to the rear of the fuel tank 7.
As shown in FIG. 5, the front wheels 4 are supported by a suspension system 9 so that each front wheel can move vertically, and the rear wheels 5 are supported by a suspension system 10 so that each rear wheel also can move vertically.
A final reduction gear mechanism (differential) 13 for driving the front wheels 4 and a final reduction gear mechanism 14 for driving the rear wheels and connected to the engine 2 via respective drive shafts 11 and 12 are provided in the center of the front portion and the center of the rear portion, respectively, of the body frame 3. The right and left front wheels 4 and the right and left rear wheels 5 are respectively connected to the final reduction gear mechanism 13 for the front wheels and the final reduction gear mechanism 14 for the rear wheels.
A two-wheel/four-wheel drive switching device for switching between rear-wheel drive and four-wheel drive by selectively transmitting and disconnecting motive power to the front wheels 4 is provided, for example, between the final reduction gear mechanism 13 for the front wheels and the drive shaft 11. Alternatively, the two-wheel/four-wheel drive switching device can be provided between the rear drive shaft 12 and the final reduction gear mechanism 14 for the rear wheels.
The switching of the mode of drive is performed by the driver of the vehicle according to factors such as the state of the road surface and the type driving.
A two-wheel/four-wheel drive switching device having the structure shown in FIG. 6 has previously been proposed.
The two-wheel/four-wheel drive switching device denoted by reference number 15 in FIG. 6 is composed of two input shaft components 16a and 16b formed by axially dividing the input shaft 16 of the final reduction gear mechanism 13 for the front wheels into two shafts which are aligned on the same axis, and a switching device 17 for selectively connecting and disconnecting the shaft components 16a and 16b. 
In more detail, a cylindrical positioning protrusion 18 is provided in the center of the end face of the input shaft component 16a located on the side of the final reduction gear mechanism 13 for the front wheels, and a positioning concave portion 19 into which the positioning protrusion 18 is fitted in such a manner that the positioning protrusion can be turned is formed in the center of the end face of the input shaft component 16b. The two input shaft components 16a and 16b can be connected so that they turn together by positioning the input shaft components 16a and 16b so that the positioning protrusion 18 and the positioning concave portion 19 are interfitted.
Splines S1 and S2 are formed on the respective outer peripheral faces of the engagement parts of both of the input shaft components 16a and 16b. The switching device 17 is arranged such that it surrounds these engagement parts.
The switching device 17 includes a switching ring 20 provided on the inner surface thereof with splines fitted to the respective engagement parts of both input shaft components 16a and 16b so that the switching ring can be slid. With this structure, the splines of the switching ring can be engaged with or disengaged from the respective splines S1 and S2 of the input shaft components 16a and 16b. The switching device 17 further includes a drive mechanism 21 composed of a solenoid for selectively moving the switching ring 20 in the axial direction of the input shaft components 16a and 16b to a position in which its splines are engaged with only one input shaft component 16a or a position in which the splines are simultaneously engaged with the splines S1 and S2 of both input shaft components 16a and 16b. 
The two-wheel/four-wheel drive switching device 15 configured as described above can thus select between a two-wheel drive mode in which only the rear wheels are driven by moving the switching ring 20 in one direction by the drive mechanism 21 so as to engage it with only the input shaft component 16a to thereby disconnect the transmission of driving force to the front wheels 4, and a four-wheel drive mode in which driving force is transmitted to both the front wheels 4 and the rear wheels 5 simultaneously by sliding the switching ring 20 so as to connect both input shaft components 16a and 16b. 
However, such a conventional type two-wheel/four-wheel drive switching device 15 has the following problems.
When there is difference in rotational speed between the front wheels 4 and the rear wheels 5, in switching drive modes engagement between the switching ring 20 and the input shaft component 16b sometimes may not be possible, thus preventing switching between modes. Even if switching can be made, there is also a problem that the splines may collide in switching, creating a clashing noise.
To solve such a problem, a mechanism for synchronizing (aligning) the splines of the switching ring 20 and the splines of the input shaft component 16b may be provided, but the structure of the switching system then becomes complex, necessitating significant changes in the layout of the existing structure.
Moreover, as the interior chamber of the two-wheel/four-wheel drive switching device 15 and that of the final reduction gear mechanism 13 for the front wheels communicate, the same lubricating oil is required to be used to lubricate the two-wheel/four-wheel drive switching device 15 and the final reduction gear mechanism 13 for the front wheels. However, as the functions of the two-wheel/four-wheel drive switching device 15 and the final reduction gear mechanism 13 for the front wheels are different, it is preferred that different types of lubricants be used.
The invention has been made to solve the problems occurring in the conventional two-wheel/four-wheel drive switching device, and an object of the invention is to provide a vehicular two-wheel/four-wheel drive switching device the configuration of which is simple, which can be employed without changing the existing configuration of the vehicle, which generates scarcely any noise, and which can be lubricated separately from other actuating parts.
A vehicular two-wheel/four-wheel drive switching device according to the invention includes a driving shaft extending to a drive side of the switching device, a driven shaft extending from a driven side of the switching device, an inner ring and an outer ring having a generally annular clearance formed therebetween, one of the inner ring and the outer ring being coupled to the driving shaft and the other of the inner ring and the outer ring being coupled to the driven shaft, a plurality of connecting/disconnecting members provided in the clearance between the inner ring and the outer ring for selectively connecting and disconnecting the inner ring and the outer ring, a switching mechanism for selectively positioning the connecting/disconnecting members in one of a position in which the inner ring and the outer ring are connected and in a position in which the inner ring and the outer ring are disconnected, a casing receiving the inner ring, the outer ring, the connecting/disconnecting members, and the switching mechanism, and first and second oil seals provided in respective locations at a predetermined interval in a longitudinal direction of the driven shaft for sealing the casing.
The vehicular two-wheel/four-wheel drive switching device according to the invention may further comprise a dust seal for sealing an end of the casing, the dust seal being provided in a clearance formed between the casing and one of the driving shaft and the driven shaft.
The inner ring has a plurality of cam surfaces formed thereon, and the outer ring has a cylindrical inner surface facing the cam surfaces.
The connecting/disconnecting members each comprise a roller, the rollers extending in the clearance between the inner ring and the outer ring in an axial direction of the driving and driven shafts.
The switching mechanism comprises a retainer for rotatably holding the rollers, and an electromagnetic clutch coupled to the retainer.
In the vehicular two-wheel/four-wheel drive switching device according the invention, in the position in which the inner ring and the outer ring are disconnected, the electromagnetic clutch is disengaged and the rollers abut lower portions of corresponding ones of the cam surfaces, while in the position in which the inner ring and the outer ring are connected, the electromagnetic clutch is engaged, the retainer moves together with the outer ring, and the rollers are moved towards higher positions of the cam surfaces so as to abut the higher portions of corresponding ones of the cam surfaces and the inner surface of the outer ring.
The outer ring may comprise first and second portions coupled to one another via a spline connection.
A bevel gear may be integrally formed with an end of the driven shaft, the bevel gear being engaged with a ring gear of a final reduction gear for the front wheels.